a All operations other than those operating under 14 CFR 121 and
14 CFR 135.

b U.S. Department of Transportation, Federal Highway Administration
(FHWA), provides data separately for passenger car and motorcycle in its annual
Highway Statistics series. However, the 1995 summary report provides
updated data for passenger car and motorcycle combined. Passenger car figures
in this table were computed by U.S. Department of Transportation, Bureau of
Transportation Statistics by subtracting the most current motorcycle figures
from the aggregate passenger car and motorcycle figures.

c In July 1997, FHWA published revised passenger-kilometers data
for the highway modes for a number of years. The major change reflected the
reassignment of some vehicles from the passenger car category to the other 2-axle
4-tire vehicle category. Passenger-kilometers for passenger car, motorcycle,
and other 2-axle 4-tire vehicles were derived by multiplying vehicle-kilometers
for these vehicles by average vehicle occupancy rates, provided by the Nationwide
Personal Transportation Survey, 1977, 1983, and 1995.

d Motor bus and demand responsive figures are also included in the
bus figure for highway.

e Prior to 1985, excludes demand responsive and most rural and smaller
systems funded via Sections 18 and 16(b)2, Federal Transit Act. The series is
not continuous between 1980 and 1985. Transit rail modes are measured in car-kilometers.
Car-kilometers measure individual vehicle-kilometers in a train. A 10-car train
traveling 1 kilometer would equal 1 train-kilometer and 10 car-kilometers.

f Amtrak began operations in 1971. Does not include contract commuter
passengers.

g Included in passenger car.

h Included in other single-unit 2-axle 6-tire or more truck.

I Includes commuter rail figures only.

j Ferryboat included in other.

NOTES

Air carrier passenger-kilometers are computed by summing the products of the
aircraft-kilometers flown on each interairport segment multiplied by the number
of passengers carried on that segment. Highway passenger-kilometers from 1960
to 1994 are calculated by multiplying vehicle-kilometers of travel as cited
by FHWA by the average number of occupants for each vehicle type. Average vehicle
occupancy rates are based on various sources, such as the Nationwide Personal
Transportation Survey, conducted by the Federal Highway Administration, and
the Truck Inventory and Use Survey, conducted by the Bureau of the Census. Transit
passenger-kilometers are the cumulative sum of the distances ridden by each
passenger. Rail passenger-kilometers represent the movement of 1 passenger for
1 kilometer.